PET-CT for — Fever of Unknown Origin

The classical definition of fever of unknown origin requires fever persisting for at least three weeks with no explanation found after a thorough initial investigation, but your doctor may order a PET-CT earlier depending on how unwell you are and what other tests have already shown. There is no single fixed rule — it is a clinical judgement made by your referring specialist based on how quickly your condition is changing and what the routine workup has revealed. The decision rests with them, not with a general threshold.

The radioactive material used is a small amount and clears from your body within hours. You will carry a low level of radioactivity for the rest of that day. Standard advice is to avoid prolonged close contact with pregnant women and very young children for that day only. You can sit in a car with other people, be around adults, and return to your usual home environment. The scan team will give you a written instruction sheet with the specific guidance before you leave.

A normal scan is genuinely useful information — it makes serious active infection, large vessel inflammation, and most lymphomas much less likely. It does not mean the fever has no cause; it means the cause is not detectable by this scan. Your specialist will use the normal result to narrow the possibilities and decide what to investigate next. No scan rules out every condition, and the interpretation depends on your full clinical picture, which only your referring doctor can assess.

You will be asked not to eat for several hours before the scan. The tracer the scan uses is a form of glucose, and eating raises blood sugar in a way that reduces image quality. Water is usually permitted. Most medicines can be taken as normal, but check with the booking team when your appointment is made — especially if you take insulin or other diabetes medication, which may need adjustment. Confirm the exact fasting duration with the imaging centre when you receive your appointment details.

Reports are typically ready within one to two working days, though the timeline depends on the centre. Your referring specialist — not the imaging team — will explain the result to you and discuss what it means for your care. If you have not heard within the time you were told to expect, contact your specialist's clinic rather than the imaging centre. The imaging team provides the report; your referring doctor is responsible for interpreting it and deciding your next steps.